This invention relates to electron guns, and especially to electron guns for use in television picture tubes. The invention is particularly directed to electron lenses for such guns, and more particularly to long focal length lenses (extended lenses) of the resistive type.
It is well known that spherical aberration in an electron lens can be desirably reduced by increasing the focal length of the lens, i.e. by making the field of the lens weaker and extending it over a greater length along the path of the beam. It is also well known that the focal length of a lens can be lengthened by increasing the size of the lens aperture and/or the gap between two electrodes of the lens. However, increasing the diameter of the lens conflicts with the desire to dispose the electron gun in a small neck of a cathode ray tube in order to minimize required deflection power; and increasing the gap between the electrodes may allow other electric fields external to the lens to penetrate the gap and distort the focus field.
The prior art has disclosed various extended lens structures designed to achieve longer focal length without the attendant disadvantages described above. One such type of extended lens is the resistive lens exemplified by FIG. 1 of U.S. Pat. No. 2,143,390 issued to F. Schroter on Jan. 10, 1939; by U.S. Pat. No. 3,932,786 issued to F. J. Campbell on Jan. 13, 1976; and by FIG. 3 of U.S. Pat. No. 4,091,144 issued to J. Dresner et al on May 23, 1978. In this type of lens, a plurality of metal electrode plates are arranged in serial fashion and a voltage gradient is established along the lens by applying different voltages to the different plates by way of a resistive bleeder element provided within the vacuum envelope of the electron tube itself. Although Schroter shows this resistor only schematically, Campbell discloses a practical embodiment of a bleeder resistor disposed on an insulator element of the electron gun structure, and Dresner et al show a practical embodiment of a stack of alternate metal electrodes and insulator blocks with a resistive bleeder coating applied along one edge of the stack. However, in practice, the Campbell structure has proved to have attendant problems of stray emission because of the many connectors required to make contact between the series of apertured electrodes and the bleeder resistor, and both the Campbell and Dresner et al lenses depend for their field accuracy upon the uniformity of the resistive bleeder coating, the fabrication of which is very difficult to control. Furthermore, neither the Campbell nor the Dresner et al lenses provide the flexibility desired for accurately shaping the lenses' voltage profile along the beam path.